This invention relates to a flow-control valve having a flow-drooping characteristic, more specifically to a valve wherein the liquid supply amount into an oil pressure mechanism is decreased in response to an increase of the amount of liquid discharge from a pump.
Flow-control valves having the drooping characteristic has been generally used for power assisted steering systems of vehicles in order to permit stabilized vehicle body steering during high speed running and also to permit steerage with excellent steerage force.
As is well known, shown by a solid line in FIG. 8 is a relationship between a steerage force required by a driver and the vehicle speed in the case of no employment of the power assisted steering system. In this case, generally, a relatively large steerage force is required, and the force is gradually reduced with the increase of the vehicle speed. Shown by a chain line in FIG. 8 is the relationship in the case of the employment of the conventional power assisted steering system. In this case a relatively smaller steerage force is required in comparison with the solid line, and in the low speed range such as city driving or garaging, such a conventional system may be appropriate, since relatively low steerage force is required in this range. However, in case of high speed running of the vehicle, the driver is dissatisfied with the light steerage force and further such light steerage force may cause the over turning of the steering wheel, resulting in the possibility of a traffic accident at high speed. Therefore a novel power assisted steering system having characteristics as shown by a broken line in FIG. 8 has been requested, which obviates the extremely reduced steerage force with the increase of the vehicle speed.
In another aspect, there is another relationship between the steerage force required by the driver and the pressure variation in the power assisted steering system due to the variation of rotation angle of the steering wheel while maintaining constant vehicle speed. That is, in the case of no employment of the power assisted steering system, the driver feels small steerage force during steerage to maintain straight running of the vehicle. However, in case of the steering wheel being rotated in accordance with the curvature of the road, a high steerage force is required. Therefore, demand has been made to provide a power assisted steering system which reduces and minimizes this steerage force differential in response to the rotation angle of the steering wheel.
In one conventional flow-control valve, generally, the amount of fluid recirculated from an outlet passage of the pump to the recirculating passage and the amount of fluid passing through the fluid supply passage into the power assisted steering system are controlled by a single spool valve. Hence, any variation of fluid pressure due to operation of the power assisted steering causes a variation of the choking amount of orifice in the fluid supply passage. This in turn causes a variation of the fluid supply amount into the power steering system in response to the fluid pressure increase during steerage.
Such a conventional flow-control device is shown in U.S. Pat. No. 3,426,785 as illustrated in FIG. 10.
This prior art patent generally shows a fluid flow-control device for power steering using a pressure actuated spool valve. The valve spool 119 is shiftable in a housing having a valve port 112, and a bypass port 121 that is connected to the inlet and outlet of the valve. The pump outlet 113 communicates with an upstream surface area of the spool 119 to urge axial bypass opening movement of the spool for increasing the bypass flow between the valve port and the bypass port. The pump outlet also communicates with the high pressure side of the steering gear via a constant metering orifice 129 and a variable metering orifice 127 and is also in fluid communication with a downstream surface area of the spool via a restricted trigger orifice 141. This cooperates with a valve biasing spring 144 to oppose bypass opening movement and as a result the pressure differential across the metering orifice is a constant determined by the reaction of the biasing spring. A pressure relief valve 150 in communication with the downstream surface area operates to exhaust fluid pressure at a maximum safe pressure that is predetermined. As a result of the restricted flow from the trigger orifice, a small pressure relief flown from the downstream surface area results in a large pressure drop and a corresponding large increase in the bypass flow.
As shown in FIG. 10, a chamber A is connected to a hydraulic pump to receive fluid therein, a chamber C is connected to a power steering system and a chamber D is connected to a bypass passage to recirculate the fluid into the pump.
The device according to this patent has the characteristics shown in FIG. 9, wherein the relationship between the rotation speed of the pump and the fluid supplying amount into the P.S. system is shown. As is evident from broken lines of FIG. 9, in case the rotation angle of the steering wheel is large (pressure in the power assisted system is large), the fluid supplying amount into the power steering system becomes large in comparison with the case in which the rotation angle of the handle is small, (pressure in the power assisted system is small), to thus complement steerage force during steerage at a high rotation angle of the wheel. Therefore, approximately uniform steerage force is obtainable regardless of the rotation angle of the handle.
However still another demand has been made to provide the novel power assisted steering system capable of providing small differential steerage force depending upon the rotation angle of the wheel. Therefore, it is necessary for the novel device to supply a given fluid amount into the power assisted steering system regardless of the vehicle speed or rotation angle of the handle.
In the above U.S. patent, referring to FIG. 10, the fluid amount flown into the power steering system is determined by the fluid pressures P.sub.1, P.sub.2, P.sub.3 (=0), and the opening areas, S.sub.1 S.sub.2, S.sub.3, of the constant metering orifice 129, variable metering orifice 127 and bypass port 121, dependent upon the movement of the spool 119, respectively.
For example, in case of no load, namely in case of substantially no rotation of the wheel, the pressure P.sub.2 does not become large as is clear from FIG. 9, so that the fluid amount flown into the power steering system is determined by the inner pressure differentials P.sub.1 -P.sub.2 and P.sub.1 -P.sub.3 and the opening area ratio S.sub.1 /S.sub.3. If the wheel is greatly rotated to increase the pressure P.sub.2, the differential P.sub.1 -P.sub.3 is larger than the differential P.sub.1 -P.sub.2, so that the valve 119 is moved toward the right to maintain the fluid amount supplied into the power steering system, resulting in that the opening area S.sub.2 of the variable metering orifice 127 becomes large to increase the fluid amount supplied into the power steering system.
Such pressure variation causes the fluid supplying amount into the power steering system to change, to thereby vary the steering force. Therefore it would be difficult to obtain stabilized steerage operation.
Another kind of valve has been proposed wherein a metering rod is disposed in the fluid supply passage connected to the power-steering system in order to control the choking amount. Such a conventional flow control valve system is shown in U.S. Pat. No. 3,314,495 and Japanese Patent Application Publication (OPI) No. 48-83530.
The valve control system uses a spool valve disposed, and movable within a hollow bore of a valve body. A spring located at one end of the bore is used to bias the spool valve. An orifice is formed in the valve body at an end opposite the spring for directing fluid from an internal hydraulic circuit inside the valve body to an external hydraulic circuit. As in other prior art arrangements the valve body has an ingress port for directing fluid from a source at increased pressure against the spool valve and a bypass port is controlled by the spool valve to a zone of reduced pressure. The spool valve is responsive to the flow rate through the orifice as represented by the pressure rate differential between the internal and external circuits. The valve system uses a metering rod that extends into the flow orifice of the valve body to reduce the effective cross-sectional area thereof for movements of the valve related to vehicular speed. This gives the valve system a drooping flow charactersitic. However, the metering rod must be machined with high accuracy to coaxially provide it into the choke opening. Similar drawbacks in terms of variation of the flow amount as mentioned above are also present in this conventional device. Further, upon axial movement of the metering rod, the tapered or stepped metering surface provides a resistant force in choking the opening so that smooth movement of the rod in response to the fluid pressure is not obtainable.